Graft fixation device

ABSTRACT

A graft fixation device comprising a sheath having a body that defines an inner longitudinal bore and a member for insertion within the bore. The sheath includes at least two projections movably connected to the sheath body by a distal hinge. The distal hinge causes the projection to pivot inwardly as the sheath is placed in a bone tunnel, and to be displaceable radially outwardly of the sheath body upon insertion of the member within the bore.

RELATED APPLICATIONS

This is a continuation-in-part of International Patent Application No.PCT/GB2008/01800, filed on May 27, 2008, which in turn claims priorityunder 35 U.S.C. §119 to U.K. Application No. 07100233, filed May 25,2007, the disclosures of which incorporated by reference in theirentirety.

BACKGROUND OF THE INVENTION

The present invention relates to graft fixation devices comprising asheath having a body that defines an inner longitudinal bore and amember for receipt within said bore. The device may be used to fix anyform of graft, for example a soft tissue graft, such as a graft ligamentor tendon, to a support structure, such as a joint with a preformed bonetunnel.

Damage to connective soft tissue, for example ligaments and tendons,often results from excessive and/or uncontrolled motion about aparticular joint, such as the knee or elbow. In severe cases, surgerycan be required to restore the joint's function by replacement orreconstruction of the damaged connective soft tissue, often using grafttissue. During such procedures the graft tissue will usually have to besecured or fixed to an appropriate support structure, such as one ormore of the bones forming part of the joint in question.

The anterior cruciate ligament (ACL), which runs from the front of thetibia to the back of the femur, helps to stabilize the knee bypreventing the tibia from moving too far forward. The demands placed onthe knee sometimes exceed its limits and if the ligament is stretchedtoo tightly, it can tear or rupture. By way of example, ACL injuries mayoccur when the upper leg is turned outward while the lower leg isturning inward. This type of injury most commonly happens to athleteswhen quickly pivoting or changing direction.

If the ACL is severely damaged it can be surgically repaired to restorethe knee's stability and function. Approximately 100,000 ACLreconstructions are performed in the US each year, and the numbers arerising. A widely used ACL repair procedure is the bone-tendon-bonepatella tendon repair procedure, although there is a growing trend forthe use of soft tissue grafts, e.g. replacing the torn ACL with ahamstring tendon graft harvested from the patient. In spite of surgicaladvances, the failure rate of ACL reconstruction is quoted to be in therange of 5 to 25%, with the main cause of graft failure being loss ofgraft fixation within the tibial bone tunnel. Most current ACL repairprocedures employ traditional interference screws to press a graftagainst a tibial and/or femoral bone tunnel wall. A disadvantage ofusing such screws is the screw thread which provides the fixation withinthe bony tunnel. The larger and sharper the thread the better thefixation, but conversely the greater the damage caused to the tendon andtherefore the risk of failure.

The pull-out strength of current fixation screws is markedly reduced incircumstances where the support structure possesses reduced bone mineraldensity. Accordingly, ACL reconstruction in middle-aged individuals israrely performed, especially in peri-menopausal women. Moreover, as ACLreconstruction is increasingly being performed on a more active olderpopulation with reduced bone quality, the ACL reconstruction failurerate is set to increase still further unless improved forms of fixationare developed.

SUMMARY OF THE INVENTION

An object of the present invention is to obviate or mitigate one or moreof the above problems and/or disadvantages. A further object is toprovide an improved device for graft fixation.

According to a first aspect of the present invention there is provided agraft fixation device comprising a sheath having a body that defines aninner longitudinal bore and a member for receipt within said bore,wherein said sheath comprises at least one projection moveably connectedto said sheath body and said at least one projection is arranged so asto be displaceable radially outwardly of said sheath body upon receiptof said member within said bore. The projections can be hinged to thesheath at either end.

By way of example only, the device can be used to fix a graft to asupport structure, such as a bone tunnel, by placing the sheath into thetunnel between strands of the graft and then inserting the member intothe bore of the sheath body. This causes outwards radial displacement ofthe projections relative to the sheath body so that they contact andcompress the graft against the tunnel wall creating strong, rigidfixation.

A second aspect of the present invention provides use of a deviceaccording to the first aspect of the present invention to fix a graft toa support structure.

The present invention provides a superior means of graft fixation thanthose currently available and removes any need for secondary fixation.Computer modeling using finite element analysis indicates that thedevice of the first aspect of the present invention will impartsignificantly greater pressure on a graft against a support structure(e.g. a bony tunnel) than conventional methods of fixation. Moreover,the modeling also predicts that the device of the first aspect of thepresent invention will fix a graft to a support structure, such as atibial bone tunnel, such that the force required to pull the graft fromthe support structure will be up to around 180% greater than the forcerequired if a traditional interference screw is used. Proof of principletests have demonstrated that even a rudimentary prototype deviceaccording to the first aspect of the present invention performssimilarly to a commercially available device. Further development of thedevice in terms of materials, and the size, shape and structure ofdetailed features should provide a range of different devices accordingto the first aspect of the present invention which are optimized fordifferent applications and outperform current commercially availabledevices. The device of the first aspect of the present invention shouldtherefore allow more aggressive rehabilitation in the young and enableconnective soft tissue reconstruction in an increasingly active olderpopulation, which is currently excluded over worries regarding inferiorbone quality.

The device of the first aspect of the present invention is eminentlysuitable for use in the fixation of anterior cruciate graft ligaments. Athird aspect of the present invention provides an anterior cruciategraft ligament fixation device comprising a sheath having a body thatdefines an inner longitudinal bore and a member for receipt within saidbore, wherein said sheath comprises at least one projection moveablyconnected to said sheath body and said at least one projection isarranged so as to be displaceable radially outwardly of said sheath bodyupon receipt of said member within said bore.

A fourth aspect of the present invention provides use of a deviceaccording to the third aspect of the present invention to fix ananterior cruciate graft ligament to a surface of bone within a tibialbone tunnel.

A fifth aspect of the present invention provides a method for fixing agraft to a support structure using a device comprising a sheath having abody that defines an inner longitudinal bore and a member for receiptwithin said bore, said sheath comprising at least one projection that ismoveably connected to said sheath body, wherein the method comprisesplacing the sheath adjacent the graft and the support structure andinserting the member into the bore of the sheath body so as to displacesaid at least one projection radially outwardly of said sheath body sothat said at least one projection contacts the graft and urges itagainst the support structure.

The device of the present invention comprises at least one projectionthat is moveably connected to the sheath body, that is, the or eachprojection is arranged such that it is moveable relative to the sheathbody rather than being fixed to the sheath body in such a manner thatit/they can only move in unison with the sheath body. This lattersituation is the case with some conventional fixation systems whichemploy expansion screws inserted into expandable sheaths carrying fixedexternal projections (as modeled in Comparative Example 2 below).

The means of connection between the or each projection and the sheathbody may be arranged to provide any desirable type of relative movement.It is preferred that said at least one projection is pivotally orhingedly connected to said sheath body. Preferably said sheath bodydefines a distal end wall and an opposite proximal end wall, said distaland proximal end walls being connected by a side wall. While the sheathbody may take any suitable form, it is preferred that the sheath body ishollow and the side wall is preferably continuous and most preferablydefines an annular cross-section. The side wall is preferably taperedradially inwardly from said proximal end wall to said distal end wall.The side wall may have any appropriate taper angle but a taper angle ofup to around 5° is preferred. More preferably the taper angle is around2° to around 5°, yet more preferably around 3° to around 4°, and mostpreferably around 3.7° to 3.9°. The sheath body preferably possesses aconsistent taper along the entire length of its side wall, starting froma diameter of about 9 mm at its proximal end and reducing to about 7 mmat its distal end. The gradient along the sheath body is equivalent to ataper angle of about 3.88° along the body (tan⁻¹ 2/30).

The proximal end wall of the sheath body preferably defines a proximalopening to permit insertion of said member into the bore of said sheathbody. The proximal end wall opening may take any size and shape providedit is appropriately dimensioned to permit insertion of the memberthrough the opening and into the bore. Preferably the opening issubstantially circular to facilitate easy and convenient insertion ofthe member.

The distal end wall of the sheath body may take any appropriateconfiguration and may be squared off, tapered or rounded. Preferably thedistal end wall of the sheath body is tapered or rounded to aidimplantation of the sheath body at the appropriate location for fixationof a graft. Most preferably the distal end wall is rounded to easeimplantation and limit any damage which the distal end wall of sheathbody may cause during implantation. It is preferred that the distal endwall defines a distal opening. The distal opening may be provided at anylocation in the distal end wall and may be of any suitable size andshape, although in a preferred embodiment, the distal end wall defines acentral circular opening. The distal end wall of the sheath body mayalso define one or more, preferably four equiangularly spaced,longitudinal slots.

In a preferred embodiment of the first aspect of the present inventionthe or each projection is outwardly radially displaceable through anaperture defined by the side wall of the sheath body. The or eachaperture may take any convenient size and shape provided it is ofsuitable configuration to enable the projection associated with thataperture to pass therethrough. The or each aperture may be circular,oval, square or of any other appropriate shape when viewed from theexterior of the sheath. In a preferred embodiment the or each apertureis substantially rectangular.

Preferably the or each projection is biased to at least partiallyproject radially outwardly of the sheath body prior to receipt of saidmember within said bore. In this configuration it may be necessary totemporarily urge the or each projection radially inwardly so that itdoes not project radially outwardly of the sheath body duringimplantation of the sheath. The or each projection may be heldtemporarily in the inwardly directed position by use of a suitableimplantation tool or the shape of a leading surface of each projectionmay be configured (e.g. tapered or rounded) such that during insertionsaid leading surface contacts a surface of the support structure (e.g.the bone tunnel) and in doing so urges the or each projection towardsthe inside of the sheath body.

Alternatively, the or each projection may be biased radially inwardly ofthe sheath body so as not to project radially outwardly of the sheathbody prior to receipt of said member within said bore. In this way, theor each projection is already retained within the sheath body so as notto hinder insertion of the sheath into the implantation site.

Preferably the or each projection comprises a graft engaging portion,which may, for example, be in the form of one or more spikes or thelike, and a connecting portion which connects the graft engaging portionto the sheath body. The connecting portion preferably comprises a distalend connected to the graft engaging portion and an opposite proximal endconnected to said sheath body. The means of connection between theproximal end of the connecting portion and the sheath body may take anysuitable form, but it is preferred that the proximal end of theconnecting portion is pivotally or hingedly connected to said sheathbody. An alternative preferred arrangement is to have the distal end ofthe connecting portion pivotally or hingedly connected to an edge of theaperture in the sheath body associated with that projection.

The or each projection may be formed separately to the sheath body ormay be integrally formed with the sheath body. Where the or eachprojection is integrally formed with the sheath body, it is preferredthat the or each projection is connected to the sheath body via a livinghinge or pivot. More specifically, it is preferred that a living hingeor pivot connects the proximal end of the connecting portion of the oreach projection to the sheath body.

It is preferred that the connecting portion of the or each projectioncomprises a surface facing towards said proximal opening of the sheathbody, said surface being arranged so as to be contactable by said memberupon insertion of the member into said bore. In this way, as the memberis inserted through the proximal opening into the bore of the sheathbody the member contacts said surface of the connecting portion of theor each projection which thereby urges that projection radiallyoutwardly of the sheath body. The graft engaging portion and saidconnecting portion of the or each projection may be formed separately ormay be integrally formed.

The device may incorporate any desirable number of projections, providedat any convenient location on the sheath. It will be appreciated thatthe particular number and arrangement of projections should be chosen tosuit the specific intended application of the device. It may besufficient to employ a device comprising a single projection.Alternatively, where the graft to be fixed comprises two strands it maybe advantageous to employ a device with at least two angularly spaced(preferably diametrically opposed) projections so that one projectioncan contact and fix one of the two strands of the graft. Thus, thedevice preferably comprises at least two projections provided atpositions that are angularly spaced about said sheath. Where graftshaving more than two strands are to be fixed it may clearly be desirableto use a device comprising more than two angularly spaced projections.For example, in the Example set out below a hamstring tendon graft isused in an ACL reconstruction procedure and the graft comprises fourstrands to be fixed. In this example, and in other procedures where itis desired to fix four or more strands it is preferred that the devicecomprises four projections provided at positions that are equiangularlyspaced about the sheath. It will, of course, be appreciated that adevice comprising two or more (e.g. four) projections may be used to fixa graft having any number of strands, i.e. one, two, three or more.While it may be advantageous, it is not absolutely necessary to select adevice having exactly the same number of angularly spaced projections asthere are graft strands to be fixed. For commercial reasons it is likelyto be most efficient to produce a range of standardized devices whichare suitable for a range of applications. In this regard, it isparticularly desirable that the device of the first aspect of thepresent invention comprises four equiangularly spaced projections.

Additionally or alternatively, the device may be provided withprojections which are spaced longitudinally along the sheath to furtherimprove the device's fixation properties. It is preferred that thedevice comprises at least two projections provided at positions that arelongitudinally spaced along said sheath. In a preferred embodiment thedevice comprises three projections provided at positions that areequally longitudinally spaced along the sheath. In further preferredembodiments, the device may comprise three, four, five or moreprojections, some or all of which may be equally spaced from oneanother.

In a preferred embodiment the or each projection is outwardly radiallydisplaceable by up to around 6 mm relative to the outer surface of thesheath body. More preferably the or each projection is outwardlyradially displaceable by up to around 4 mm and most preferably by up toaround 2 mm. In preferred embodiments where the device of the presentinvention incorporates diametrically opposite projections, it will beappreciated that if each projection can displace radially outwardly byup to around, for example, 2 mm, then the overall diameter of the devicewill increase by up to around 4 mm due to radial displacement of thespikes.

The sheath body may be formed of any appropriate material bestowing thebody with any desirable characteristic. It is preferred that the sheathbody is deformable radially outwardly upon receipt of said member withinsaid bore. The degree to which the sheath body is deformable orexpandable may be chosen to suit a particular application. Thus, in someapplications it may be desirable to use a sheath body formed from a moreflexible material which will facilitate greater radial deformation,whereas in some other applications it may be advantageous to employ asheath body formed from a less flexible material which will exhibitlower radial deformation.

Additionally or alternatively, outward radial expansion of the sheathbody upon insertion of the member may also be facilitated by theprovision of suitable slots, apertures, openings, channels or the likedefined by the sheath body. The aperture(s) defined by the side wall ofthe sheath body associated with the projection(s) may be configured tofacilitate outward radial deformation or expansion of the sheath bodyupon insertion of the member into the bore of the sheath. Said expansionmay arise, at least in part, due to deformation of one or more wall ofthe aperture(s). The aperture(s) associated with the projection(s) andany other slots, opening etc which are provided may be configured tofacilitate outward radial expansion of the sheath body by up to around 2mm (equating to a total increase in cross sectional diameter of thesheath body of up to around 4 mm), or more preferably up to around 1 mm(equating to a total increase in cross sectional diameter of the sheathbody of up to around 2 mm).

In a preferred embodiment, the or each projection is capable of beingdisplaced radially outwardly of the sheath body by up to around 6 mm andthe sheath body is capable of expanding radially outwardly by up toaround 2 mm. Thus, in this preferred embodiment, the overall diameter ofthe sheath is capable of increasing by up to around 16 mm (12 mm due tothe projections and 4 mm due to the sheath body) when the sheath body isfully expanded and diametrically opposite spikes are fully outwardlydisplaced. In a further preferred embodiment, the or each projection iscapable of displacing radially outwardly by up to around 2 mm and thesheath body can expand radially by up to around 1 mm, providing a devicein which the cross sectional diameter of the device can increase by upto around 6 mm (4 mm due to the projections and 2 mm due to the sheathbody) upon insertion of the member within the inner longitudinal bore ofthe sheath.

The sheath body of the prototype device used in some of the testsdescribed in the Examples section below was manufactured from relativelyinexpensive nylon purely for convenience. The sheath body may in fact bemanufactured from any suitable material that is currently employed forgraft fixation devices. It is preferred that at least one of said sheathbody, projection(s) and member is formed of a surgical grade plasticmaterial. It is particularly preferred that the sheath body and the oreach projection are formed from a surgical grade plastic material.

The surgical grade plastic material may be selected from the groupconsisting of polyetheretherketone, polylactic acid, polyglycolic acid,polycaprolactone, poly(lactic-co-glycolic acid),poly(glycolide-co-caprolactone), poly (glycolide-co-trimethylenecarbonate) and mixtures and blends thereof.

At least one of said sheath body, projection(s) and member is preferablyformed of a bioabsorbable and/or biodegradable material.

With regard to the member, it is preferred that it is formed of asuitable plastic, or may be formed from any other appropriate material,such as a surgical grade metal, e.g. stainless steel. The member ispreferably solid, but may be at least partially hollow. Preferably themember defines a distal end wall and an opposite proximal end wall, saiddistal and proximal end walls being connected by a side wall. The sidewall is preferably tapered radially inwardly from said proximal end wallto said distal end wall, with a preferred taper angle approximatelymatching the taper angle of the sheath body. It is preferred that themember has a taper angle of up to around 5°, more preferably around 2°to around 5°, yet more preferably around 3° to around 4°, and mostpreferably around 3.7° to 3.9°. The member preferably possesses aconsistent taper along the entire length of its side wall, starting froma diameter of about 9 mm at its proximal end and reducing to about 7 mmat its distal end. The gradient along the member is equivalent to ataper angle of about 3.88° along the member (tan⁻¹ ⅔O).

The side wall of the member is preferably continuous and most preferablydefines a screw thread, which may run continuously over the entirelength of the side wall, or just a portion thereof. An inner surface ofthe sheath body which is arranged to contact the member as the member isinserted is preferably provided with a screw thread which iscomplementary to the screw thread defined by the side wall of themember. The inner surface of the sheath body may define a screw threadalong substantially its full length and/or circumference, or just one ormore portions of the inner surface may define a screw thread orroughened surface. It is desirable that the member, when in the form ofa screw or partial screw is finished or polished to round-off any sharpedges on the thread profile and produce a smooth, uniform surfacefinish. At least a portion of the inner surface of the sheath body maydefine a suitably roughened surface to form an interference fit with theside wall of the member as it is inserted into the longitudinal bore ofthe sheath.

Additionally or alternatively, the side wall of the member may define asubstantially smooth or flat surface. In preferred embodiments of themember in which the side wall of the member tapers from one end to theopposite end, at least a portion of the side wall of the member mayextend substantially linearly between said ends so as to define asubstantially constant taper along said portion of the side wall of themember, or said portion may curve radially inwardly (to define a concaveportion) or radially outwardly (to define a convex portion) between saidends so as to define a varying taper along said portion of the sidewall. It will be appreciated that the side wall of the member may defineany number of said linear and/or curved portions extending along thelength of the member and/or around the circumference of the member. Aninner surface of the sheath body, which is arranged to contact themember as the member is inserted, preferably defines a profile which iscomplementary to the profile of the side wall of the member.

In preferred embodiments, the member has a wedge or peg-like formdefining opposite side walls which are linear substantially throughouttheir length from one end of the member to the opposite end of themember. Where the sheath body is intended to be used with a wedge orpeg-like member the inner surface of the sheath body may define acomplementary substantially smooth continuous surfaces alongsubstantially its full length, or just one or more portions of the innersurface may define a substantially smooth continuous surface.

The device according to the first aspect of the present invention maycomprise a sheath body and member provided with appropriate features todefine a ratchet-like mechanism comprising a plurality of teeth on onecomponent which are engaged by a complementary pawl defined by the othercomponent. In a preferred embodiment, the teeth are defined by theinternal surface of the sheath body and the member (e.g in the form of awedge or peg) represents the pawl. In this way, as the member isinserted the sheath body and associated projections are displacedradially outwardly, but the member is then hindered from being withdrawnfrom the sheath due to engagement of component representing pawl (e.g.the member) by the component representing the teeth (e.g. the sheathbody).

In a preferred embodiment of the device of the first aspect of thepresent invention a diameter of at least one of the proximal end wall ofthe member and the proximal end wall of the sheath body is about 2 mm toabout 6 mm. A longitudinal length of at least one of the sheath body andthe member may be about 6 mm to about 18 mm. It is particularlypreferred that a member having a proximal end wall of diameter about 2mm to about 6 mm has a longitudinal length of about 6 mm to about 18 mm.Moreover, it is preferred that a sheath body having a proximal end wallof diameter about 2 mm to about 6 mm has a longitudinal length of about6 mm to about 18 mm.

Smaller diameter devices (2-6 mm×6-18 mm) can be used for proceduressuch as biceps tenodesis, elbow collateral ligament stabilization andmedial patello-femoral ligament reconstruction.

In a further preferred embodiment of the device of the first aspect ofthe present invention a diameter of at least one of the proximal endwall of the member and the proximal end wall of the sheath body is about6 mm to about 15 mm. A longitudinal length of at least one of the sheathbody and the member may be about 20 mm to about 35 mm. It isparticularly preferred that a member having a proximal end wall ofdiameter about 6 mm to about 15 mm has a longitudinal length of about 20mm to about 35 mm. Moreover, it is preferred that a sheath body having aproximal end wall of diameter about 6 mm to about 15 mm has alongitudinal length of about 20 mm to about 35 mm.

Larger diameter devices (6-15 mm×20-35 mm) can be used for applicationswhere a bulky tendon/synthetic graft is used e.g. Anterior and PosteriorCruciate Ligament Reconstruction and Knee collateral ligamentreconstruction/repair.

It may be desirable to produce devices incorporating sheath body inthree ‘standard’ sizes: 6-7 mm diameter×30 mm length, 8-9 mm×30 mm and10-11 mm×30 mm.

It is preferred that the diameter of the proximal end wall of the memberis similar to the external diameter of the proximal end wall of thesheath body. It is more preferred that the diameter of the proximal endwall of the member is substantially the same as the external diameter ofthe proximal end wall of the sheath body. Preferably a longitudinallength of the sheath body is similar to a longitudinal length of saidmember. More preferably a longitudinal length of the sheath body issubstantially the same as a longitudinal length of the member. In thisway, as the member is inserted or screwed into the bore of the sheathbody the sheath body is caused to expand or deform, preferably radiallyoutwards, to accommodate the member. As discussed above, expansion ordeformation of the sheath body may be facilitated by forming the sheathbody from a material having the desired physical characteristics and/orproviding appropriate slots, openings, apertures and/or channels in thesheath body.

The proximal and distal end walls of the member may take any convenientform to facilitate insertion of the member into the inner longitudinalbore defined by the sheath body. Preferably, the proximal end walldefines a formation for engagement by a suitable driving tool, such as ascrewdriver, to drive the member into the longitudinal bore. By way ofexample, the proximal end wall may define a slot extending transverse tothe longitudinal bore for engagement by a slot-headed screwdriver, ahexagonal depression for engagement by a hex key, or a crosseddepression for engagement by a cross-headed screwdriver. The distal endwall of the member may define a substantially flat cross section, or acurved cross section, such that, for example, the member has anoutwardly domed or convexly curved distal end wall.

As stated above, the second aspect of the present invention provides useof a device according to the first aspect of the present invention tofix a graft to a support structure, such as, but not limited to, aportion of bone, e.g. a portion of bone within a bone tunnel.

From the foregoing description of the device and methods forming thedifferent aspects of the present invention it will be appreciated thatthe following advantages are provided: each tendon bundle or strand canbe reliably captured or fixed by the device; the graft construct is bothstable and rigid; up to 360 degrees graft to bone compression strengthcan be provided which provides enhanced concentric healing and increasedfixation strength. Provision of the device of the present invention indifferent sizes facilitates maximization of tendon-to-bone contact. Nodebris is left behind after surgery, the surgical learning curve isshort and relatively simple and the inventive device and methodseliminate post-op hardware removal from the surgical site.

The fifth aspect of the present invention provides a method for fixing agraft to a support structure using a device comprising a sheath having abody that defines an inner longitudinal bore and a member for receiptwithin said bore, said sheath comprising at least one projection that ismoveably connected to said sheath body. The method comprises placing thesheath adjacent the graft and the support structure and inserting themember into the bore of the sheath body so as to displace said at leastone projection radially outwardly of said sheath body so that said atleast one projection contacts the graft and urges it against the supportstructure.

It will be appreciated that the device forming the first aspect of thepresent invention is eminently suitable for application in the methodforming the fifth aspect of the present invention and that preferredembodiments of the device as set out above may be used in the abovedefined fifth aspect of the present invention.

With respect to the device to be used in the fifth aspect of the presentinvention it is preferred that the at least one projection is pivotallyor hingedly connected to the sheath body. Preferably the or eachprojection is biased to at least partially project radially outwardly ofthe sheath body prior to receipt of said member within said bore.Alternatively, the or each projection may be biased radially inwardly ofthe sheath body so as not to project radially outwardly of the sheathbody prior to receipt of said member within said bore.

Preferably the method further comprises forming a tunnel in the supportstructure and locating the graft in said tunnel prior to placing thesheath adjacent the graft. In a further preferred embodiment the methodfurther comprises aligning the projection(s) of the sheath with a regionor regions of the graft which are to be fixed to the support structureprior to insertion of the member into the bore of the sheath body.

The support structure may be a bone tunnel, which may be a bone tunnelformed in a tibia.

The method forming the fifth aspect of the presenting invention ispreferably used in an anterior cruciate ligament reconstructionprocedure to fix an anterior cruciate ligament graft to a wall of atibial bone tunnel.

A preferred surgical method for implanting a device according to thefirst aspect of the present invention is as follows. The method set outbelow describes the fixation of an anterior cruciate ligament graft to atibial bone tunnel but it will be appreciated that the general approachmay be adapted to fix any desirable graft to any form of supportstructure.

A tibial bone tunnel may be drilled with the use of an appropriateaiming guide. A guide wire may be placed so that it exits at the ACLfootprint. An appropriately sized cannulated drill may then be chosendependent on the graft size, and used to create the tibial tunnel. Theuse of impaction drills may improve the performance of the fixation. Thetibial tunnel may be created initially using a drill, preferably ofapproximately 7 mm diameter. This may be followed by a larger diameterdrill (e.g. an 8.5 mm diameter drill). This is intended to provide asmall clearance (e.g. approx. 1.5 mm) between a sheath (e.g. 7 mmdiameter sheath) and the tunnel wall. A tapered punch may then beinserted to dilate the tunnel to, for example, 10.5 mm, at the tunnel'sentry point and a narrower diameter, for example, 8.5 mm, at the end ofthe punch. The punch may be marked to allow the punched hole to besubstantially the same length as the member to be inserted into thesheath. It is desirable to use the punch so that there is a smallclearance of, for example, 1.5 mm between the sheath and tunnel over thelength of the sheath. Use of the punch may also advantageously increasethe bone density at the bone tunnel wall and thereby the performance ofthe device of the present invention.

The femoral tunnel may then be created through the tibial tunnel usingthe appropriately selected off-set guide. Initially a guide wire isplaced using the guide and the previously selected cannulated drill usedto create the femoral bone tunnel.

A four strand looped hamstring graft (or allograft/synthetic graft) maythen be pulled into the femoral tunnel through the tibial tunnel andfixed with the Surgeon's choice of recognized methods of fixation.

It is desirable that the graft is tensioned and cycled before fixation.Initially the sheath forming part of the device of the present inventionmay be inserted using an introducer or insertion tool so that theprojections are aligned with the four strands of the graft. The sheathmay then be fully inserted before the member is inserted into the sheathat which point the projections are displaced radially outwardly so as tocontact and compress the graft strands against the tibial tunnel wall.Fixation of the graft may also be assisted by outward radial expansionof the sheath body upon insertion of the member into the sheath. Thisexpansion is facilitated in part by the apertures defined by the sheathassociated with the projections and in part by the flexible propertiesof the material from which the sheath is manufactured. Typically, thelevel of outward radial expansion of the sheath body may be up to around1 mm, which equates to a total increase in cross sectional diameter ofthe sheath body of up to around 2 mm.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be further described by way of example only withreference to the following non-limiting Example, Comparative Examplesand preferred embodiment of the present invention, with reference to theaccompanying figures in which:

FIGS. 1( a)-(c) is a computer simulation illustrating a device accordingto the first aspect of the present invention at various stages ofinsertion of the member (shown side on) into the bore of the sheath body(shown in cross section in FIG. 1( a) and side on in FIGS. 1( b) and1(c));

FIG. 2 is a plot comparing experimental and modeled results for pull-outforce (mean value) versus displacement for a bovine flexor tendon graftfixed in a porcine tibia bone using a standard interference screw, whichdemonstrates the accuracy of the computer model developed to comparedthe performance of the device of the present invention to the standardinterference screw and the Intrafix™ system;

FIG. 3 (a) is a computer simulation of stress levels experienced by adevice according to a first aspect of the present invention and a bonetunnel wall during a graft fixation as part of an ACL reconstructionprocedure. Higher levels of stress on the device and bone tunnel aremore lightly coloured and lower levels are more darkly coloured;

FIG. 3( b) is a finite element model of the device of FIG. 3 (a) showingoutward radial displacement of projections during insertion of themember into the sheath body. The same colour coding is used as in FIG.3( a);

FIG. 4( a) is a computer simulation of stress levels experienced by aconventional fixation screw and a bone tunnel wall during a graftfixation as part of an ACL reconstruction procedure. The same colourcoding is used as in FIG. 3( a);

FIG. 4( b) is a computer simulation of stress levels experienced by aconventional fixation screw at discrete stages of insertion of the screwinto a tibial bone tunnel (shown in cross section) as a part of an ACLreconstruction procedure. The same colour coding is used as in FIG. 3(a); FIG. 4( c) is a finite element model of the screw of FIGS. 4( a) and4(b) illustrating stress levels experienced during insertion of thescrew as a part of an ACL reconstruction procedure. The same colourcoding is used as in FIG. 3( a);

FIG. 5 is a computer simulation of stress levels experienced by a priorart fixation system (Intrafix™) which employs a sheath and an expansionmember, and a bone tunnel wall during a graft fixation as part of an ACLreconstruction procedure. The same colour coding is used as in FIG. 3(a);

FIG. 6 are schematic drawings of a member and sheath according to thepresent invention showing the dimensions of various features in mm;

FIG. 7 is a graph comparing tunnel wall stress during fixation for thedevice of FIG. 3 (a) (“Spiked plug”), the screw fastener of FIG. 4( a)(“Interference screw”) and the fixation system of FIG. 5( a)(“Intrafix”);

FIG. 8 is a graph prepared using results from the computer modelingwhich compares the pull-out forces required to remove a tendon graftfrom a bone tunnel for the inventive device of FIG. 3( a) (“Spikedplug”), the screw fastener of FIG. 4( a) (“Interference screw”) and thefixation system of FIG. 5( a) (“Intrafix”);

FIGS. 9( a)-(c) show another sheath embodiment according to theinvention having distally connected projections spaced on the sheathbody and a member for insertion into the sheath body.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIGS. 1( a)-(c) there is shown a graft fixation device 1comprising a sheath 2 and a screw 3 according to the first aspect of thepresent invention in various stages of assembly. The sheath 2 is 30 mmlong and defines a proximal end wall 4 of 9 mm diameter and a distal endwall 5 of 7 mm maximum diameter separated by a continuous annular sidewall 6. The side wall 6 is linearly tapered from the proximal end wall 4to the distal end wall 5 with a taper angle of 3.88° (tan⁻¹ 2/30). Theproximal end wall 4 defines a proximal opening 7 of 7 mm diameter whichis of sufficient dimension to facilitate insertion of the screw 3through the opening 7 into a longitudinal bore 8 defined by the body 9of the sheath 2.

The screw 3 has the same overall external dimensions as the sheath 2,that is the screw is 30 mm long and tapers linearly from its proximalend 10 (diameter 9 mm) to its distal end 11 (diameter 7 mm). In anotherembodiment the screw is relatively straight from its proximal end 10 toits distal end 11. The proximal end 10 of the screw 3 is provided with aslot (not visible in FIGS. 1( a) to 1(c)) for engagement by a suitabledriving tool, such as a screwdriver, to drive the screw 3 into thelongitudinal bore 8 of the sheath 2. It will be appreciated that theslot could be replaced by any suitable formation, such as a hexagonaldepression for engagement by a hex key, or a crossed depression forengagement by a cross-headed screwdriver, to enable the screw 3 to beinserted into the longitudinal bore 8. The distal end 11 of the screw 3may take any convenient form to facilitate insertion into the sheath 2,in the specific embodiment shown in FIGS. 1( a) to 1(c) the distal end11 of the screw 3 has a relatively flat cross section, but in furtherpreferred embodiments the screw 3 may have a domed or convexly curveddistal end 11.

The distal end wall 5 of the sheath 2 is rounded and defines a centralcircular distal opening 12 and a plurality of equiangularly spacedlongitudinal slots 13.

The side wall 6 of the sheath 2 defines a plurality of rectangularapertures 14 each of which is suitably dimensioned to enable aprojection 15 associated with that aperture 14 to move radially in andout of the sheath body 9. The apertures 14 are also configured to deformso as to facilitate outward radial expansion of the sheath body uponinsertion of the member 3. The embodiment shown in FIGS. 1( a) to 1(c)incorporates twelve apertures 14 and associated projections 15 arrangedin four equiangularly spaced sets of three longitudinally spacedapertures/projections 14/15. Within each set of threeapertures/projections 14/15 the apertures 14 are equally longitudinallyspaced by 4.5 mm. The aperture 14 closest to the distal end wall 5 ofthe sheath body 9 is 4.0 mm long and the two remaining apertures 14 are4.5 mm long. The aperture 14 closest to the distal end wall 5 of thesheath body is longitudinally spaced 2.0 mm from the distal end wall 5,and the aperture 14 closest to the proximal end wall 4 is longitudinallyspaced 3.0 mm from the proximal end wall 4.

Each projection 15 comprises a graft engaging portion 16, whichterminates with a pointed tip 17, and a connecting portion 18 whichconnects the graft engaging portion 16 to the body 9 of the sheath 2.The point of connection of each connecting portion 18 to the sheath body9 defines a living hinge 19 at a proximal edge 20 of the aperture 14.Each connecting portion 18 defines a surface 21 which faces towards theproximal opening 7 and is arranged in this way so that upon insertion ofthe screw 3 into the bore 8 the screw 3 contacts the surface 21 of eachprojection 15 and urges that projection 15 radially outwardly of thesheath body 9 through its associated aperture 14. Within each set ofthree longitudinally spaced projections 15, the surface 21 of theprojection 15 closest to the distal end wall 5 of the sheath body 9 is3.5 mm long and the corresponding surfaces 21 of the two remainingprojections 15 are both 4.0 mm long.

When the screw 3 is screwed into the bore 8 of the sheath body 9, asshown in FIGS. 1( b) and 1(c), the screw 3 bears against the surfaces 21of each group of four equiangularly spaced projections 15 in turn,thereby urging each group of four projections 15 radially outwardly ofthe sheath body 9 as shown in FIG. 1( c). When the group of fourprojections 15 closest to the proximal end wall 4 of the sheath body 9are fully radially extended they provide a maximum cross sectionaldiameter of 14 mm. When the central group of four projections 15 arefull extended they provide a maximum cross sectional diameter of 13 mm,and the projections 15 nearest the distal end wall 5 of the sheath body9 provide a maximum cross sectional diameter of 12 mm.

FIGS. 9( a)-(c) show a further embodiment of the graft fixation device1, in which the projections are attached to the sheath by a hinge attheir distal end, rather than their proximal end. As shown in FIG. 9(a), the graft fixation device 1 includes the sheath 2 and the screw 3.The sheath 2 defines the proximal end wall 4 and the distal end wall 5separated by the body 9. Though shown substantially circular incross-section, sheath 2 could have other cross-sectional configurations,e.g., pentagonal, hexagonal, oval or any shape that would facilitategraft fixation. The body 9 defines a continuous annular side wall 36 anda cap 30, but for the aperture 14, discussed below. The side wall 36 islinearly tapered from the proximal end wall 4 to the cap 30 with a taperangle of 1°. In other embodiments, the side wall 36 has a taper angle of2°, 3°, 4° or does not have a taper angle. The cap 30 defines a proximalcap end 31 and a distal cap end 32 and is conical in shape, having arelatively flat surface 35 at the distal cap end 32. The cap distal end32 is also the distal end 5 of the sheath 2. The proximal cap end 31continuously connects to the sidewall 36. The cap 30 further has a capopening 33 located at the center of the relatively flat surface 35 atthe distal cap end 32 that is circular in shape. The proximal end wall 4defines the proximal opening 7, which is of sufficient dimension tofacilitate insertion of the screw 3 through the opening 7 into thelongitudinal bore 8 defined by the body 9 of the sheath 2.

The side wall 36 of the sheath 2 defines the plurality of rectangularapertures 14 each of which is suitably dimensioned to enable theprojection 15 associated with that aperture 14 to move radially in andout of the side wall 36. The apertures 14 are also configured to deformso as to facilitate outward radial expansion of the side wall 36 uponinsertion of the member 3. The embodiment shown in FIGS. 9( a) to 9(c)incorporates twelve apertures 14 and associated projections 15. Inanother embodiment, the sheath 2 incorporates from one to elevenapertures 14 and associated projections 15. Apertures 14 thoughillustrated to be substantially rectangular in shape could have othergeometries, e.g., oblong, elliptical, polygonal, hexagonal, octagonal,etc.

In FIG. 9( b) the twelve apertures 14 are shown in three sets ofapertures 140, 141, 142, wherein each aperture set 140-42 contains fourapertures 14 that have the same longitudinal placement along the sidewall 36. Aperture set 140 is placed near the proximal end 4, set 142 isplaced near the distal end 5 and set 141 is placed between set 140 and142 on the side wall 36. The apertures 14 within each set 140-142 areequiangularly spaced along the side wall 36. In another embodiment theapertures 14 in each set 140-142 are not spaced equiangularly along theside wall 36.

Further, the apertures 14 within each aperture set 140-142 arecircumferentially offset along the side wall 36 with respect to theapertures 14 in another set 140-142. For example, the apertures 14 inset 140 have the same longitudinal placement and are circumferentiallyspaced apart by the same distance, but are circumferentially offset by30° with respect to the apertures 14 in set 141 and arecircumferentially offset by 60° with respect to the apertures 14 in set142. Thereby the apertures 14 in set 141 are circumferentially offset by30° along the sidewall 36 with respect to the apertures in set 142.

The projections 15 are arranged along the side wall 36 in the samemanner as the apertures 14. Having sets of projections 15circumferentially offset facilitates contact between the projections 15and the graft strands (not shown).

Each projection 15 comprises a graft engaging portion 44 and aconnecting portion 42 which connects the graft engaging portion 44 tothe side wall 36 of the sheath body 9. Each graft engaging portion 44defines a plurality of spikes 45 and valleys 46, wherein the spikes 45are configured to engage and compress the graft against the bone tunnelwall thereby fixing the graft in place. Each projection could have otherengagement portion on its outer surfaced capable of engaging andcompressing the graft. The point of connection of each connectingportion 42 to the side wall 36 defines a living hinge 43 at a distaledge 40 of the associated aperture 14.

FIG. 9( c) shows the longitudinal bore 8 of the sheath 2. Eachprojection 15 defines a surface 41. The surface 41 of each projection 15is relatively within the same plane of the sheath body 9. When thesheath 2 is placed within a tunnel (not shown) with graft strands, thesides of the tunnel and the graft strands bear against each projection15. Thus, the projections 15 are urged radially inwardly of the sheathbody 9 through its associated aperture 14. The surfaces 41 of eachprojection 15 are thus displaced and no longer in the same plane of thesheath body 9. This inward movement helps to eliminate any damage to thegraft strands as the sheath 2 is placed within the tunnel.

When the screw 3 is screwed into the bore 8 of the sheath body 9, asshown in FIG. 9( b), the screw 3 bears against the inwardly displacedsurfaces 41 of each projection 15, thereby urging each projection 15radially outwardly of the sheath body 9.

An anterior cruciate ligament (ACL) reconstruction employing the deviceof the present invention begins by drilling a tibial bone tunnel withthe use of an appropriate aiming guide. A guide wire is placed so thatit exits at the ACL footprint. An appropriately sized cannulated drillis chosen (dependent on the graft size) and is used to create the tibialtunnel. The use of impaction drills improves the performance of thefixation. The tibial tunnel is created initially using a 7 mm diameterdrill followed by an 8.5 mm diameter drill. This is intended to providea 1.5 mm clearance between a sheath having a 7 mm distal diameter andthe tunnel wall. A tapered punch is then inserted to dilate the entrypoint of the tunnel to 10.5 mm leaving the distal end of the tunnel at8.5 mm diameter. The punch is marked to allow the punched hole to be thesame length as the screw which will be inserted into the sheath. It isdesirable to use the punch so that there is a small clearance of 1.5 mmbetween the sheath and tunnel over the length of the sheath.

The femoral tunnel is then created through the tibial tunnel using theappropriately selected off-set guide. Initially a guide wire is placedusing the guide and the previously selected cannulated drill used tocreate the femoral bone tunnel.

A four strand looped hamstring graft is then pulled into the femoraltunnel through the tibial tunnel and fixed with the Surgeon's choice ofrecognized methods of fixation.

The graft should be pre-tensioned, cycled and tensioned before fixation.Initially the sheath is inserted using an introducer so that theprojections are aligned with the four strands of the graft. In anotherembodiment, the sheath has projections displaced around the entirecircumference of the sheath body thus eliminating the need to align theprojections with the graft strands. The sheath is then fully insertedbefore the screw is inserted into the sheath. As the screw is insertedinto the sheath the projections are displaced radially outwardly so asto contact and compress the graft strands against the tibial tunnel walland thereby fix the graft in place.

EXAMPLES

Experimental studies on animal models and computer simulations werecarried out in the context of an ACL reconstruction. The experimentaltests provided basic material data which was needed to understand thefailure modes of ACL reconstruction, as well as to provide informationneeded for modeling and verification of the simulation. The computermodeling provided technical information which could not have beenobtained as easily via pure experimental investigation, such as stressdistributions, interfacial loading between the ligament graft,sheath/screw and bones, and first point of failure. The simulation alsoprovided a means to allow various parameters to be tested so as tooptimize the design of the fixation device of the present inventionwithout requiring extensive experimental investigation. Experimentaltests were then carried out to determine the performance of annonoptimized prototype device according to the present invention forcomparison to a commercially available device.

To setup the ACL reconstruction, a tunnel was drilled into a fresh pigor calf tibial bone using a clinical drill bit. A bovine flexor tendonwas passed through the tunnel and fixed using standard metal and plasticinterference screws. Some of the specimens were sectioned in a frozenstate along the axial direction to observe the tibial tunnel structureand interfaces of the fixation into the tibia tunnel. This provideddetailed information on the geometric profile for computer modeling. Forexample, the length and cross-sectional area of the specimens were usedfor strain and stress calculations. Additionally, non-destructiveexamination of the ACL reconstruction using x-ray photography wascarried out.

The tendons were tested using screw-driven computer controlled universaltensile test machines. The specimen's response to the loading wasobtained in the form of a load-displacement curve. FIG. 2 shows a plotof pull-out force (mean value) versus displacement for a test which usedporcine tibial bone and a calf tendon graft. A 10.5 mm tunnel wasdrilled into the porcine tibial bone. The tendon graft with a totaldiameter size of 7 mm was inserted into the tunnel with the assistanceof sutures hanging from the strands. A loop of tendon approximately 40mm long was left extending out from the upper part of the tibial bone.The loop was used to hold the graft to a hook in the upper grip of thetesting machine.

The sheath forming part of the device of the present invention wasplaced at the entrance of the bone tunnel and was inserted with the helpof a special driver, sliding it between the tendon strands until it wasin a position similar to that of the interference screw. The projections(in the form of spikes) connected to the sheath body were retainedinside the sheath during insertion. With the sheath inside the bonetunnel, the member (in the form of a tapered screw) was inserted intothe inner longitudinal bore of the sheath to expand the sheath in aradial direction and displace the spikes radially outwardly of thesheath body. The tibial bone tail was fixed to the base and the tendongraft was pulled in-line with the axes of the tunnel. The specimen wasfixed to the testing machine and the tendon graft then subjected to astandard pull-out test at a rate of 20 mm/min until failure.

In parallel, 3D numerical modeling (explained more fully below) wasperformed to mimic the experiment. For the sake of the numericalmodeling it was assumed that the device did not contact the bone duringfixation. The result from the model is also presented in FIG. 2 forcomparison.

The agreement between modeled and experimental values was goodconsidering the assumptions made on loading conditions, geometry andmaterial properties and the exclusion of the mechanical influence of themuscles, ligaments and cartilage. The model has provided an insight intothe type and magnitude of the forces acting on the bone in ACLreconstruction which has enabled the prediction of patterns of thestress, strain and displacement in the regions around the bone tunnel.

A commercial code, ABAQUS/Explicit, was used to simulate the dynamicturning and advancing of the screw and radial outward displacement ofthe projections. The bone was modeled using a 3-dimensional solidelements C3D4 element, a 4-node linear tetrahedron provided by the code.This element has the advantage of providing high accuracy in geometricmeshing for complicated 3D shapes, which is required for the contactbetween the bone and the fixation device during the fixation process.Mesh adequacy was validated using a convergence analyses. This wasachieved using a standard method whereby the results of severaldifferent mesh densities were compared to ensure they were sufficientlyclose to each other. In this way it was possible to establish anappropriate mesh density which was fine enough, but not so fine as torequire extra calculation with effectively no enhancement in theaccuracy of the model. The screw was also modeled using the solidelements C3D4 element.

The finite element model was created for a section of tibial bonecontaining a tunnel and an inserted interference screw, Intrafix™sheath, and sheath forming part of the device of the present invention.The materials for cortical, cancellous and subchondral bones wereassumed linear elastic, which is adequate for most studies of bonestress and strain. The modeled bone was assigned a stiffness value fromexperimental data. The shear modulus was calculated from empiricalrelationships reported by Ashman et al. and an isotropic Poisson's ratiowas used. Due to scarcity of experimental data, subchondral bone wasassumed as isotropic and homogeneous.

In the model, a 9 mm diameter tunnel was created at locations reportedby Fu et al.³, i.e. at an angle of about 10° to the midsagittal planeand 45° to the midcoronal plane. The locations were within theboundaries of those used clinically.

The screw or sheath was inserted with a force directed along the tunnelaxis (i.e. longitudinally), which has to be equal to the graft tensionat full extension of the knee during gait.

The cortical and cancellous bone properties of stiffness and shearmodulus were derived from a combination of experimental data obtainedduring the current study and the open literature.^(9,10,11) The stressesin at the interface between the bone and the screw or sheath wereexamined at different stages of fixation.

FIG. 3 (a) is a computer simulation of stress levels experienced by adevice according to a first aspect of the present invention and a bonetunnel wall during a graft fixation as part of an ACL reconstructionprocedure. Higher levels of stress on the device and bone tunnel aremore lightly colored and lower levels are more darkly colored. FIG. 3(b) is a finite element model of the device of FIG. 3( a) showing outwardradial displacement of projections during insertion of the member intothe sheath body.

Experimental tests using a standard testing protocol were conducted todetermine the fixation performance of a rudimentary prototype deviceaccording to the present invention so that its performance could becompared to a commercially available Intrafix™ device.

The prototype device according to the present invention was manufacturedusing non-optimised materials (nylon), components (i.e. size and shapeof sheath and member) and manufacturing techniques. The prototype devicetherefore provided only an initial indication of the potentialcapabilities of the device. Testing was carried out using six samples ofthe prototype device and six samples of the Intrafix™ device usingporcine tibial bone and bovine flexor tendon graft.

COMPARATIVE EXAMPLES

Computer simulations of two different prior art methods of screwfixation have been carried out. Several models have been developed whichfacilitated the simulations of the installation of the screw, thedeformation of the ligament, as well as the distribution of the stressin both the bone and the ligament to be analyzed. Using actualexperimental data allowed the input of the modeling to be updated so asto allow for more accurate simulation of the mechanical environment.Comparative tests were then carried out to compare the performance of annonoptimized prototype device according to the present invention to acommercially available device.

Comparative Example 1

FIG. 4( a) is a computer simulation of stress levels experienced by aconventional fixation screw and a bone tunnel wall during a graftfixation as part of an ACL reconstruction procedure. The same colorcoding is used as in FIG. 3( a). FIG. 4( b) is a computer simulation ofthe stages of insertion of a conventional fixation screw into a tibialtunnel during an ACL reconstruction. FIG. 4( c) is a finite elementmodel of the screw of FIGS. 4( a) and 4(b) illustrating stress levelsexperienced during insertion of the screw as a part of an ACLreconstruction procedure.

Comparative Example 2

Intrafix™ from DePuy Mitek (a Johnson & Johnson company) is a softtissue tibial fixation system for anterior cruciate ligamentreconstruction. The Intrafix™ system has been in clinical use since1999. The Intrafix™ (non-absorbable) and bio-Intrafix™ soft tissuefasteners are patented tibial fixation systems, designed to maximize thestrength and stiffness of an ACL reconstruction using soft tissuegrafts. FIG. 5 is a computer simulation of stress levels experienced byan Intrafix™ fixation system and a bone tunnel wall during graftfixation as part of an ACL reconstruction procedure. The same colorcoding is used as in FIG. 3( a).

Comparative Example 3

A further computer model was developed to compare the fixation strengthof the interference screw, Intrafix and device of the present invention.The same boundary condition and failure mode were assumed for all threetypes of device. Therefore, the results provide useful comparativevalues rather than providing actual numerical data for the devices whichcould be compared to experiment. For this reason a 2D axisymetric finiteelement explicit method was used in this analysis.

For all three types of device an 8 mm cylindrical tunnel was created.The diameter of the tunnel was chosen to be smaller than the maximumdiameter of the fixation device to cover the differences between theshapes of the devices.

The same material property was assumed for the tibial bone tunnels(based on the human bone). For the device of the present invention,stainless steel and Nylon were defined for the member and sheathrespectively. The materials were modeled as exhibiting uniform linearelastic behaviour. No viscosity or yield/failure was defined for thematerials in these tests.

The external boundary of the tunnel was restricted in the axialdirection and the boundary assumed non-reflective.

An interaction (penalty contact method) was defined between the bone anddevice. The friction coefficient between the screw to bone; sheath tobone; and member to sheath was assumed as 0.35, 0.3 and 0.25respectively. A linear displacement boundary equal to the tunnel lengthwas defined for the screw, pin and sheath to fully place inside thetunnel. No rotational motion was assumed for the screw, pin or sheathduring insertion.

The interference screw was modeled with a rounded pitch (no sharp edges)but the tapered screw in the Intrafix™ and the member used with theinventive device was assumed as a smooth pin to simplify the analysis.

Comparative Example 4

Cyclic-loading testing evaluates the cyclic behaviour of agraft-fixation construct, and thus, better allows the determination ofimmediate post-operative changes which may occur.

In order to compare the dynamic strength of an interference screwfixation method with a device of the present invention, several porcinetibial bones with bovine flexor tendon graft were cyclically tested.

Tendon grafts were fixed into bone tunnels with 35 mm long metalinterference screws or devices according to the present invention(dimensions as shown in FIG. 6). A preload of 5 N was used beforerepetitive loading in the direction parallel to the long axis of thebone tunnel. The specimens were loaded cyclically with the loading axisin line with the graft-bone tunnel at a frequency of 1.0 Hz. The graftwas then cycled in two stages as follows.

First, each graft was loaded in tension from 50 N to 150 N (100+−50 N)at the different number of cycles. After loading in each cycle, thegraft was unloaded and allowed to rest for 5 seconds before reloading.This was followed by measurement of the graft displacement. In a 1000cycle loading test, the construct was unloaded in 1 second and leftunder zero load for 60 seconds before reloading, to allow time-dependentrecovery of the graft. Second, each graft was loaded at 200 to 400 N(300+−100 N) with a different number of cycles followed with a 5 secondrest and measuring the graft displacement.

After conducting each cyclic loading test, the graft was subjected to aload to failure under a constant rate of displacement parallel to theaxis of the grafts if the fixation had survived.

The results of these tests are presented below in Table 2.

Comparative Results

FIG. 7 is a graph of the results generated in Comparative Example 3above, which compares tunnel wall stress during fixation for the deviceof the present invention (“Spiked plug”) to the conventional screwfastener from Comparative Example 1 (“Interference screw”) and theIntrafix™ fixation system of Comparative Example 2. The minimum level ofstress on the tunnel wall was provided by the conventional screw and themaximum level of stress was provided by the device of the presentinvention, which indicates that the device of the present inventionforced the graft ligament against the bony tunnel with the greatestforce of the three fixation methods under comparison.

FIG. 8 is a graph comparing the pull-out forces required to remove thegraft from a bone tunnel for the device of the present invention(“Spiked plug”), the standard interference screw fastener fromComparative Example 1 and the Intrafix™ fixation system of ComparativeExample 2. The maximum pull-out force exhibited by any of the threefixation systems under comparison was just over 700 N, which wasexhibited by the device of the present invention (see Table 1). Thislevel of force is up to 180% higher than the standard interference screwand up to 130% higher than the level exhibited by the Intrafix™ system.The maximum pull-out force exhibited by the device of the presentinvention is also significantly greater than the threshold pull outforce of 500 N, which ACL surgeons recommend.

TABLE 1 Maximum Fixation System pull-out force Percentage Screw 422.7 N  100% Intrafix ™ 578.6 N 129.8% Inventive Device 757.7 N 179.3%

The results of the cyclic loading tests are presented below in Table 2.In conclusion, all grafts employing the conventional screw fixationmethod failed at the fixation site with the tendon being pulled past thescrew when the initial loading of the second round cyclic loading wasapplied. In contrast, when the device of the present invention was usedthe specimen survived the cyclic loading and so a load to failure couldbe applied. The load at failure was over 500 N for all of the testscarried out using devices according to the present invention. It wasalso observed during the cyclic loading tests that the applied force atinitial slippage for each of the graft strands was significantly higherwhen using the device of the present invention compared to theconventional interference screw.

TABLE 2 Graft elongation (mm) Number of Interference Inventive CyclesScrew Device 150 N (100 ± 50 N) Loading at 1 Hz 10 0.90 0.53 50 1.710.87 100 1.80 1.27 200 2.08 1.52 300 2.21 1.80 400 2.26 1.92 500 2.421.97 750 2.68 2.29 1000 2.82 2.44 400 N (300 ± 100 N) Loading at 1 Hz 106.81 fixation 3.47 failed 50 — 3.89 100 — 4.43 200 — 4.74 300 — 4.91 400— 5.19 500 — 5.35 750 — 5.68 1000 — 5.88

The results presented below in Table 3 compare the performance of theprototype device according to the present invention to a commerciallyavailable Intrafix™ device in standard tests conducted to determine theability of the devices to fix a bovine flexor tendon graft to porcinetibial bone.

It can be observed from the results that even a rudimentary unoptimisedprototype device according to the present invention performsapproximately as well as the commercially available product (i.e. therewas no statistically significant difference in the performance of thetwo types of devices), suggesting that with further development, thedevice according to the present invention is likely to outperformcurrent commercially available devices, such as the Intrafix™ device.

TABLE 3 Inventive Device Load Ultimate Required for Yield load to*Cyclical 5 mm tendon Point failure Stiffness Displacement DisplacementSample (N) (N) (N/mm) (mm) (N) Mode of Failure 1 992 992 379 1.69 911tendon slipped past fixation 2 935 935 286 2.36 768 tendon slipped pastfixation 3 1227 1227 263 2.79 718 tendon slipped past fixation 4 10321032 245 2.30 753 tendon slipped past fixation 5 1553 1553 235 2..57 736tendon slipped past fixation 6 1181 1217 235 2.89 700 tendon slippedpast fixation Average 1153 1159 274 2.43 764 St. Dev 226 227 55 0.43 76IntraFix ™ Device Average 1271 1376 256 2.73 791 St. Dev 317 407 58 0.29156 (*displacement for the tendon after 500 cycles from 0 to 400 Napplied load)

Although the present invention has been described in relation toparticular embodiments thereof, many other variations and modificationsand other uses will become apparent to those skilled in the art. It ispreferred, therefore, that the present invention be limited not by thespecific disclosure herein.

What is claimed as new and desired to be protected by Letters Patent ofthe United States is:
 1. A method for fixing strands of a graft to asupport structure, comprising the steps of: providing a devicecomprising a sheath having a body that defines an inner longitudinalbore and a tapered screw for receipt within said bore, said sheathcomprising at least two longitudinally spaced projections each havingproximal and distal ends with a distal end hingedly and movablyconnected to said sheath body, the projections being movably connectedto the body of the sheath by a distal hinge that causes the projectionsto pivot inwardly as the sheath is placed into a tunnel in the supportstructure, and to be displaceable outwardly of the body of the sheathupon insertion of the tapered screw, each projection comprising a graftengaging portion and a connecting portion which connects the graftengaging portion to a side wall of the body of the sheath, each graftengaging portion defining a plurality of spikes and valleys, the spikesbeing configured to engage and compress the graft against a wall of thetunnel in the support structure; forming the tunnel in the supportstructure; subsequently, inserting the graft strands in the tunnel;subsequently, placing the sheath at an entrance of the tunnel andsliding the sheath in between the graft strands, and further fullyinserting the sheath into the tunnel adjacent the inserted graft strandsso as to displace the projections radially inwardly of said sheath body;and subsequently, inserting the tapered screw through an opening in thesheath body and into the bore of the sheath body so as to displace theprojections radially outwardly of said sheath body so that theprojections contact the graft strands and compress and urge the graftagainst walls of the tunnel in the support structure.
 2. The methodaccording to claim 1, wherein the projections are hinged to the sheathbody distally with respect to the sheath opening that accepts thetapered screw.
 3. The method according to claim 1, wherein there arefirst, second, third, and fourth projections and the first and secondprojections have the same longitudinal spacing which is different fromthe longitudinal spacing of the third and fourth projections.
 4. Themethod according to claim 3, wherein the first and second projectionsare equally spaced circumferentially about said sheath, but arecircumferentially offset about said sheath from the third and fourthprojections.
 5. The method according to claim 1, wherein said supportstructure is a bone tunnel.
 6. The method according to claim 1, whereinsaid tunnel is formed in a tibia.
 7. The method according to claim 1,wherein said method is an anterior cruciate ligament reconstructionprocedure and said graft strands form a 4-stranded anterior cruciateligament graft being fixed to a wall of a tibial bone tunnel.